1. Field of the Invention
The present invention relates to a method for detecting radar objects with the aid of a radar sensor of a motor vehicle, in which a measuring signal is generated by mixing a transmitted signal with a received signal, the transmitted signal having a sequence of frequency modulations.
2. Description of the Related Art
Radar sensors are used in driver assistance systems for motor vehicles, for example, in ACC (adaptive cruise control) systems or safety systems, for example, collision warning systems, and are used for the purpose of determining the positions and relative velocities of objects, so that a driving situation may be recognized. For example, in the case of an ACC system within the scope of adaptive cruise control, the velocity of one's own vehicle may be adapted to the velocity of a preceding vehicle and the distance to the preceding vehicle may be regulated to a suitable value. For example, a safety system may be configured for the purpose of automatically initiating a braking procedure if the risk of an immediately imminent collision is recognized.
FMCW (frequency-modulated continuous wave) radar sensors are known, in which the transmission frequency of a continuous radar signal is modulated in ramps. A baseband signal to be analyzed, which is analyzed, is generated from a received signal by mixing with the transmitted signal. A radar sensor typically has multiple channels, which each provide a baseband signal.
In the case of an FMCW radar sensor, each radar object in the frequency spectrum of a channel emerges in the form of a peak, whose location is a function of the Doppler shift and the runtime of the radar signals, so that unambiguous determination of the relative velocity and the distance is not yet possible from the baseband signal assigned to a frequency modulation ramp. Rather, the frequency of an obtained peak establishes a relationship between relative velocity ν and distance d in the form of a linear relationship.
The term “linear” is understood hereafter to mean that the relationship thus designated may include a linear factor and an additive term.
In the case of the FMCW method, multiple frequency modulation ramps having different ramp slopes are necessary to identify multiple radar objects and estimate their relative velocities and distances. By comparing the different relationships obtained in the case of the individual frequency ramps, relative velocity ν and distance d of a radar object may be calculated. This comparison is also referred to as matching and corresponds to a search for intersection points of straight lines in the ν-d space. The FMCW method is particularly efficient if only a few radar objects are detected.
Radar sensors are also known which operate according to the method of chirp sequence modulation, in which the transmitted signal includes a sequence of identical, frequency-modulated signal pulses (chirps). This is a pulse Doppler method, in which initially a separation of the radar objects according to their distances takes place and subsequently, on the basis of the differences of phase relations between the reflections of the individual signal pulses, location changes and therefore velocities of the radar objects are ascertained.
Such a method is known from Published German patent application document DE 10 2005 048 209 A1.
Published international PCT patent application document WO 2011/066993 A2 also describes such a method. Three measuring cycles are carried out successively, each having different, limited unambiguous ranges both for the determination of the distance and also for the determination of the velocity. To determine a measured value for the relative velocity, a search is made for corresponding values of the measured values, which are ascertained ambiguously in each measuring cycle, for the velocity. To determine a measured value for the distance, a search is made for corresponding measured values of the various measuring cycles, the object being filtered out as an out-of-range object if no correspondence is found within an unambiguous range of a first measuring cycle.
In order to achieve precision of the determination of the relative velocity and the distance in the chirp sequence method, a high number of steep signal pulses and therefore a great computing effort during the analysis of the measurements are necessary.